The purpose of providing ribs on the exterior surface of a plastic pipe is to strengthen the wall of the pipe. It is therefore important that the construction of the ribs be made in a manner to ensure maximum strength of the ribs.
A preferred method of making a ribbed pipe is through the use of a molding apparatus having a die which feeds to mold blocks which move past and receive molten plastic from a plastic feed passage of the die. It is known to angle the feed passage relative to the mold blocks to set the best direction in which the molten plastic is fed onto the faces of the mold block. For instance, Corma Inc. of Toronto, Ontario, Canada has designed a die with a plastic flow passage angled at about 45° relative to the axis of the die. This passage feeds the molten plastic at the 45° angle onto faces of ribbed pipe forming mold blocks which move parallel to the surface of the die.